first things first: Thanks for your excellent work on Discourse. We are running an instance on http://community.hiveeyes.org/ and choosing Discourse was one of the best things which could happen to us. Keep up the spirit!
While running this forum, we had one minor hiccup the other day with Discourse stuck on "Currently Upgrading" we have been able to resolve quickly.
A few minutes ago, we had another one when attempting to upgrade Discourse using
launcher rebuild app on a slightly loaded machine: We experienced the very same behavior as @ljpp had with Upgrade failed due to unclean database shutdown.